This invention concerns equipment and methods for x-ray mammography. More specifically this invention concerns equipment and methods for positioning and supporting patients during tomographic mammography procedures.
Cancer of the breast is a major cause of death among the American female population. Effective treatment of this desease is most readily accomplished following early detection of malignant tumors. Major efforts are presently under way to provide mass screening of the population for symptoms of breast tumors. Such screening efforts will require sophisticated, automated equipment to reliably accomplish the detection process.
The x-ray absorption density resolution of present photographic x-ray methods is insufficient to provide reliable early detection of malignant breast tumors. Research has indicated that the probability of metastatis increases sharply for breast tumors over 1 cm in size. Tumors of this size rarely produce sufficient contrast in a mammogram to be detectable. To produce detectable contrast in photographic mammograms 2 to 3 cm dimensions are required. Calcium deposits used for inferential detection of tumors in conventional mammography also appear to be associated with tumors of large size. For those reasons, photographic mammography has been relatively ineffective in the detection of this condition.
A method of computerized x-ray tomography has recently been developed which is capable of providing greatly increased sensitivity for the detection of tumors in soft tissue. Variations of tomographic x-ray methods have, for example, been described in U.S. Pat. Nos. 3,778,614 and 3,881,110 to Hounsfield and others. The method generally involves the electrical detection and recording of a plurality of x-ray views taken along multiple transmission paths through tissue. The views are then combined in suitable computing machinery to provide detailed cross sections of the internal tissue structure.
The calculation of image data from tomographic information is greatly simplified if the x-ray absorbtion along the various transmission paths is approximately equal. It has, therefore, been a common practice to immerse tissue undergoing tomographic examination in a symmetrical mass of fluid. The above-referenced Hounsfield U.S. Pat. No. 3,881,110 describes a structure for immersing body parts in such a fluid.
The time required for mammographic x-ray exposures may be substantially reduced by use of a diverging x-ray beam and a multiple detector array. The efficient calculation of image information produced in such a configuration, however, requires that views be taken about a 360.degree. arc surrounding the tissue.
Approximately 50 percent of breast tumors are known to occur in the upper, outer quadrant of the breast. A significant number of breast tumors also occur within approximately 1 cm of the skin. Prior art mammographic methods were generally inadequate to detect these tumors.